Radiation levels within three of the reactor buildings at the Fukushima Nuclear plant in Japan are still too high for people to start decommissioning the reactors, two years on from the second-worst nuclear disaster in history.

Scientists still do not have a firm understanding of the precise conditions of the reactor cores in three of the six units at the Fukushima Daiichi plant, and are resorting to using remote-controlled vehicles to get inside the tangle of wires, pipes and rubbles that has lain untouched since the tsunami tore through the facility.

Workers wearing protective suits and masks are seen near the No.4 reactor (EPA)

The Tokyo Electric Power Co, the plant's operator, insists that much has been achieved to bring the situation at the reactors under control. Radiation levels are declining, work is under way to build a crane that will be able to remove the spent fuel rods from the No. 4 unit at the plant and the debris is being cleared away.

For all the upbeat assessments emerging from TEPCO however, no one has been able to spend more than a couple of minutes at a time inside the three reactor buildings since they were crippled by the massive earthquake and tsunami on March 11, 2011, which killed almost 16,000 people nationwide, and the scale of the problem in those units is still not clear.

"I would like to make a sincere apology to people all over the world for the concern that we have caused at the Daiichi plant," Takeshi Takahashi, the head of the crippled plant, said.

"Radiation levels at units one, two and three are very high and the cause of that is the fuel that has melted inside the reactors," he said.

The only success that the 3,000 workers who flood into the plant every day have had so far is with reactor 4, and that is largely because the unit was undergoing an inspection at the time of the disaster and its 1,533 fuel rods were stored in the spent fuel pool out of immediate harm's way.

TEPCO has played down fears about the safety of the pool, which is on the third floor of a building badly damaged by the earthquake and tsunami, although extensive remedial and reinforcement work has been carried out. Some nuclear experts have warned that another major tremor could bring down the pool and expose the rods to the air, which would be an even greater catastrophe than has already befallen north-east Japan.

Workers carry out radiation screening on bus for media tour (Reuters)

To remove that threat, TEPCO is constructing a huge new tower alongside the building that will house a crane designed to lift the rods into casks and then lower them to ground level where they can be transported to a secure site, Mr Takahashi said. The crane should be ready to go into operation in November, he said.

Nonetheless, Mr Takahashi agrees with the assessment that it will take at least 30 years before the plant can be fully decommissioned.

The situation around the other reactor buildings remains no less chaotic.

The twisted hulk of a truck that was caught in the tsunami is still rammed up against the shell of one of the reactor buildings. Steel barriers have been bent perpendicular to the ground by the force of the waves that battered the plant.

Teams of workers in regulation white or blue protective all-body suits go about their duties, further protected by three layers of gloves, two layers of socks, a skull cap and an uncomfortable full face mask with respirators.

"Radiation has no colour or smell and if you work in that environment for a while you get used to it and you're not afraid any more," admitted Hiroshige Kobayashi, 45, the head of the construction office at the plant for contractor Kajima Corp. "That's a psychological thing among the workers here, but we constantly try to remind them that there is a threat to their health, to enhance their awareness.

"Before I got here, I imagined how difficult the environment was going to be and my team was worried. But now we have come here and seen that everything is being done under strict controls, so the risk is not as bad as we had imagined.

"Our people have a sense of mission in their hearts and they want to do something," he said. "Our aim is to return this place to how it was before so that the local residents can come back to their homes."

The 18-mile exclusion zone from Fukushima is still vigorously enforced by police roadblocks. But even beyond that mandatory boundary, communities that were thriving two years ago are today little more than ghost towns. Shops, banks and post offices are shuttered, weeds are sprouting through the pavements and homes that look brand new have been abandoned, kids' bicycles left in the driveways.